Process for the deposition of resinous films on aluminum-bearing substrates

ABSTRACT

A PROCESS IS DESCRIBED FOR FORMING ON AN ANODIZED POROUS ALUMINUM OR ALUMINUM ALLOY SURFACE A RESINOUS PROTECTIVE FILM WHICH IS APPLIED WHILE THE SURFACE IS WET AND MOIST SUBSEQUENT TO WATER-WASH.

United States Patent Int. Cl. B4411 1/34 US. Cl. 117-49 6 Claims ABSTRACT OF THE DISCLOSURE A process is described for forming on an anodized porous aluminum or aluminum alloy surface a resinous protective film which film is applied while the surface is wet and moist subsequent to water-wash.

SUMMARY OF INVENTION This invention relates to a method of producing on a metal surface a protective, ornamental film. More specifically, the invention relates to a process of this character which permits the formation on an aluminum or aluminum alloy surface a smooth, level and strong resinous film by immersion-deposit of a coating.

For low cost and easy mass workability, aluminum alloys find wide utility in fabricating therefrom a variety of useful commercial articles including window sashes, panels and the like.

A number of processes have hitherto been proposed for treating and finishing aluminum-bearing substrates so that product aluminum articles are resistant to environmental attack, i.e. corrosion by acidic or alkaline atmosphere and present a continuous, smooth, level surface appearance. Prior to the application of a protective coating and/ or an ornamental dye, the aluminum surface is anodized and treated to fill or level out the fine pores or cavities which develop in the surface when anodized. The surface is water-rinsed and dried, and thereafter coated by spraying, or electro-deposition with suitable coatings such as solutions of normally dry acrylic resins, thermo-setting acrylic resins, polyurethane resins, watersoluble acrylic resins and the like.

The porosity-filling treatment, which follows the step of anodization and which is usually made with steam to form gamma-Al O -H o or gamma-Al O -3H O film on the aluminum surface, is intended to prevent entry of corrosive materials into anodized porous surface. After this treatment, the aluminum surface is washed with water and required to be dried when applying thereto the aforesaid normally dry acrylic resins, thermo-setting acrylic resins or polyurethane resins. The Water-soluble acrylic resin coatings are most commonly applied by means of electro-deposition which requires costly equipment and complicated operating procedures including severe pH control, coating potential adjustment and high-purity water requirement.

Whereas, it is an object of the present invention to provide a new, useful process which will eliminate or alleviate the foregoing disadvantages encountered with the prior art processes for the production of coated films on aluminum or aluminum alloy substances.

It is a specific object of the invention to provide a 3,711,313 Patented Jan. 16, 1973 simple and economical process for forming on an aluminum or aluminum alloy surface a smooth, level and firmly adhered film.

These objects and other features of this invention will be apparent from the following description taken in connection with certain embodiments of the invention.

As a result of extensive research of the relationship between the aluminum surface characteristics and the coating compositions to be applied thereon, said relationship having a bearing upon the quality of the film, it has been found that the above-mentioned objects of the invention may be achieved by the use of certain watersoluble, thermo-setting resin coating compositions and of certain process steps that appear hereinafter.

Briefly stated, the inventive concept of this invention resides in a process wherein an anodized aluminum or aluminum alloy surface is immersion-deposited with a water-soluble thermo-setting resin coating composition, set for a While and dried by heating at optimum temperature. More specifically, the process according to the invention is characterized in that a starting aluminum surface, after degreasing, cleaning,- waterwashing and anodizing is deposited with a desired film while the surface is still Wet and moist, i.e. Without first being dried. This is because the coating compositions have great affinity to water, and the water present on the aluminum surface is readily miscible with the coatings and, as the coated surface is heated, vaporizes or has part of its particle attached to aluminum oxide on the surface thereby forming A1 0 H O (boehmite) Where anodized aluminum is desired to be coated, the aluminum surface may be coated also While it is wet upon water-washing and rather without further porosity-filling treatment which has been employed in the art to eliminate numerous pores or cavities developed in the anodized surface and Which has heretofore been considered essential to avoid corrosion attack. Such porosity-filling effect is more effectively achieved by the process of the invention wherein residual water, co-present with appreciable amounts of sulfuric acid radicals and other anion materials resulting from anodization of an aluminum surface and remaining in the anodized porous surface, becomes miscible with a water-soluble, thermo-setting coating and vaporizes or forms an Al O -H O film which in turn provides an increase in the volume of the aluminum surface area sufiicient to fill the cavities and at the same time, assists in the adhesion of the coating composition to the aluminum surface.

The water-soluble, thermo-setting resins employed in accordance with the invention include acrylic resins, alkyd resins, acrylalkyd resins and any one of these resins as admixed with amino resins. A successful amino resin example is melamine which may be blended with acrylic resins, alkyd resins or acrylalkyd resins at a ratio of 10-40 to 60-90 percent in solid state.

Preferred coating compositions according to the invention typically comprise parts by weight of any one of the resin examples given above, 65-170 parts of watersoluble organic solvent such as alcohol or ether alcohol, 35-60 parts of Water, 0.1-0.5 part of anion-type surfactant and about 7.05 parts of organic amine (stabilizer), the blending proportions being such that the solid content be of the order of 30-50% (of the mass upon heating and hardening the coating composition.

When applying the coating compositions to an object in the practice of the invention, there should be further added about 30 parts of water and 20 parts of suitable diluents so that the solid resin deposit on the applied object when heated and dried is about 20-35%.

Typical examples of the invention coating compositions are given in the following table:

TABLE Coat No.

70 parts 70 parts 65 parts acrylaciylic alky alkyd 100 and and and 100 100 parts 30 parts 35 parts 30 parts parts parts aciylmelamelamela- Resin acrylic alkyd alkyl mine mine mine Butanol 12 12 12 12 IPA 22 20 20 22 20 20 Ethylene glycol mono butyl ether- 90 80 80 90 80 80 Water 30 3O 30 8O 30 3O Anion surfactant- 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 Amine 7. 5 7. 5 7.5 7. 5 7. 5 7. 5

N orE.The figures appearing in the foregoing table are parts by weight.

Typically, the process according to the invention comprises applying to an aluminum or aluminum alloy surface a water-soluble thermo-setting resin coating of the type and composition shown in the above table, causing the coated surface to set for about 1-10 minutes, and drying and hardening the surface at temperature ranging from 150 C. to 200 C. for about 15-30 minutes.

The following examples are provided to further illustrate the process of the present invention, but these are not to be regarded as limiting.

EXAMPLE I An aluminum workpiece was subjected to pretreatment in which it was degreased and cleaned by dipping it for 1 minutes in a 5% emulsion-type detergent heated to 70 C.; washed with room temperature water; etched by dipping it for 3 minutes in a 5% NaOH solution maintained at 80 C.; washed with Water; neutralized by dipping for 2 minutes in a 10% NHO solution at room temperature; and washed again with water. The pretreated substrate was subjected to anodization using a H 80 electrolyte operated with a DC 16 volts source at a current density of 1 a./dm. at a bath temperature of 20 C. The substrate was thus anodized for 30 minutes. It was thereafter washed with water at two difierent time intervals, and while still wet, immersed for 1 minute in an immersion bath containing Coat #2. The substrate was taken out of the bath and disposed to set over a period of 10 minutes. The coated aluminum surface was then heated in an electrical furnace at a temperature of 180 C. for 20 minutes, until it was dried and hardened.

One of the important objects of the invention is, as

previously stated, to provide a firmly adhered protective film on an aluminum or aluminum alloy surface. To achieve this specific object; i.e. to deposit a desired coated film on an aluminum surface with high adhesion, the present invention contemplates the utilization of the numerous fine pits or pores which develop in the surface of an aluminum bearing substrate as this is electrically anodized in a conventional manner. These pores are known to be generally in the range of 100-600 angstroms. On the assumption, therefore, that such pores may be substantially filled by a relatively low polymerization resin coating which coating is of the same composition as a relatively high polymerization resin coating, it has been found that excellent results may be obtained by applying to anodized porous aluminum surface a first coating having a molecular weight not exceeding 1,000 and then, a second. coating of similar composition having a molecular weight of the order of 3,000-4,000, both first and second coatings being of the same type and composition as exemplified in the table above. The first low polymerization resin coating is believed to penetrate into the pores in anodized aluminum surface and thus firmly anchored therein when it hardens. Since the first coating is of the same composition as the second coating, both may be readily miscible and may form a continuous, uniform film on an aluminum surface. Suitable pigments, if a colored surface finish is desired, may be admixed with the first low polymerization undercoating which uudercoating may be retained intact, should the second high polymerization coating layer be weathered or worn with time.

EXAMPLE II EXAMPLE III The procedure of Example II was repeated except that 0.5 g./l. of a red color organic pigment was admixed with the first low polymerization resin coating which was warmed at 30 C. and inwhich the aluminum workpiece was immersed for 10 minutes. I

In accordance with the invention, there is provided alternatively a process for producing a firmly adhered resin film on an aluminum or aluminum-bearing substrate. This process consists in applying to an immersion bath an ultrasonic vibration in the range of 20-50 he in such a manner that the coating in the bath undergoes cavitation which results in accelerated activity of the particle of the coating composition and which at the same time causes the water and other anion materials residual in the pores of the anodized aluminum surface to move out and replace with the coating. The cavitation accompanies the formation of air bubbles which may be eliminated by the use of a less viscous coating having a viscosity of about 15 seconds by Fordcup test #4, or by adding a suitable defoaming agent. Alternatively, such air bubbles may be eliminated by discontinuing the application of ultrasonic vibration to the immersion bath immediately before the workpiece is withdrawn, or by allowing the coating to overflow, or by providing two separate baths, one for ultrasonic vibration and the other for finish-coating.

EXAMPLE IV Coat #4 was adjusted to a viscosity of 15 seconds at 30 C. by Fordcup test #4 and applied by immersiondeposit with ultrasonic vibration of 20 kc. to anodized porous aluminum surface for 5 minutes. The aluminum workpiece was then re-immersed in a similar coating without vibration for 30 seconds. The coated workpiece was allowed to set over 10 minutes and dried at a temperature of C. for 20 minutes.

EXAMPLE V The procedure of Example IV was followed except that the ultrasonic vibration Was 30 kc.

EXAMPLE VI The procedure of Example IV was followed except that the ultrasonic vibration was 25 kc. and the coating viscosity was 20 seconds at 30 C. by Fordcup test #4.

Although the present invention has been illustrated by reference to specific examples, it will be understood that such various changes and modifications thereof will be apparent to those skilled in the art as fall within the scope of the appended claims.

What is claimed is:

1. A process of coating an aluminum or aluminum alloy surface which comprises:

(a) degreasing, cleaning and water-rinsing the surface;

(b) etching the water-rinsed surface with an alkaline solution;

() neutralizing and water-washing the etched surface;

(d) anodizing the surface, whereby pits and cavities are formed in the surface;

(e) water-rinsing the anodized surface;

(f) without prior filling of said pits and cavities and while the anodized surface is still wet, applying a resin coating composition to the surface, said composition comprising a water-soluble, thermosetting resin selected from the group consisting of acrylic resins, alkyd resins and acrylalkyd resins and having a solid content of about 20-35 percent by weight;

(g) allowing the resin to set for about 1-10 minutes;

and

(h) drying and hardening the coated surface at a temperature of about 150-200 C. for 15-30 minutes.

2. A process for forming on an aluminum or aluminum alloy surface a smooth, level and strong resinous film, which comprises:

(a) subjecting an aluminum workpiece to a pretreatment in which the workpiece is degreased and cleaned by dipping it into an emulsion-type detergent, washed with water, etched by dipping it into a NaOH solution, water-Washed, neutralized by dipping it into a HNO solution, and again water-washed;

(b) anodizing the workpiece whereby pits and cavities are formed in the surface of the workpiece, and water-rinsing the anodized workpiece;

(c) applying to the workpiece surface, while still wet and moist and without prior filling of the pits and cavities, a resin coating composition consisting of 100 parts by weight of a water-soluble, thermosetting resin selected from the group consisting of acrylic resins, alkyd resins and acrylalkyd resins, 65-170 parts of alcohol, 3560 parts of water, 0.1- 0.5 part of anionic surfactant and small amounts of organic amine, said composition being adjusted to contain 20-35 percent in solids; and

(d) hardening the applied coating thus obtained for about 15-30 minutes at a temperature of about 150- 200 C.

3. A process as claimed in claim 2, wherein the coating composition is applied onto the wet anodized aluminum surface by immersion of the latter into the coating composition.

4. A process as claimed in claim 3, wherein the coating is applied under ultrasonic vibration in the range of 2050 kc.

5. A process as claimed in claim 2, wherein step (c) is carried out by first immersing the wet workpiece into a first bath of said coating composition, wherein the resin has a molecular weight not exceeding 1,000, permitting the coating thus formed to set and then immersing the coated workpiece into a second bath of said coating composition, wherein the resin has a molecular weight of about 3,000-4,000.

6. A process as claimed in claim 5, wherein the first coating composition contains a pigment.

References Cited UNITED STATES PATENTS 3,053,691 9/1962 Hartman 117-132 C X 3,544,499 12/1970 Hatch 117-132 B 3,298,852 1/1967 Beatty et al. 117-118 X 2,541,901 2/1957 Zademach et al. 117-49 X 2,614,912 10/1952 Rice 117-49 X 2,134,319 10/1938 Stone 117-49 3,413,158 11/1968 Inouye et al. 117-132 B X 2,979,417 4/1961 Kriiger etal 118-57 2,231,373 2/1941 Schenk 204- N OTHER REFERENCES Condensed Chemical Dictionary, 6th ed. Rheinhold, New York, N.Y., 1961, p. 1202.

ALFRED L. LEAVITT, Primary Examiner J. A. BELL, Assistant Examiner US. Cl. X.R.

117-75,109, 132 B; 204-35 N, 38 A 

